(a) Field of the Invention
The present invention relates to a synchronization method in a distributed wireless communication system and a terminal supporting the same.
(b) Description of the Related Art
Wireless communication systems may be basically divided into a synchronous systems and an asynchronous systems.
A synchronous wireless communication system refers to a system in which terminals operate in accordance with a common reference time. In a synchronous wireless communication system, efficiency of the usage of radio resources can be improved and a system having excellent performance can be easily designed because a specific operation is performed at a predetermined time. Furthermore, power consumption of a terminal can be reduced through a power consumption reduction mode. For example, terminals operating in the power consumption reduction mode may operate in a reception mode only during a predetermined time, and the reception units of the terminals may be powered off in the remaining time, thereby being capable of reducing power consumption. A terminal which wants to send data to a terminal operating in the power consumption reduction mode may send the data or a control message to release the terminal from power consumption reduction model during a predetermined reception mode time.
Synchronous wireless communication systems may be divided into systems using a centralized synchronization method and systems using a distributed synchronization method. A representative example of systems using the centralized synchronization method includes a cellular system. In a cellular system, terminals within a cell perform synchronization based on synchronization signals (or timing reference signals) provided by a base station. In the distributed synchronization method, synchronization signals are not transmitted by a specific device, but terminals within a network participate in the transmission of synchronization signals to achieve synchronization. Such a distributed synchronization method is suitable for a device-to-device (D2D) communications network or an ad hoc network.
In an asynchronous wireless communication system, terminals perform transmission and reception operations without a predetermined reference time. In an asynchronous wireless communication system, terminals always monitor a radio channel in order to receive packets from other terminals within a network. If a packet is detected during the monitoring, terminals estimate the starting point of the packet using a preamble signal included in the packet and read information in the packet. An asynchronization wireless communication system can be easily implemented because there is no predetermined reference time, and is chiefly used in systems whose resource efficiency does not need to be high. However, there is a disadvantage in that power consumption is high because terminals need to always monitor a radio channel.
One of the distributed synchronization method may include the following method. Each terminal periodically sends a synchronization signal using its own synchronization timing (i.e., reference time). After receiving synchronization signals from surrounding terminals, each terminal performs its own time synchronization using the received synchronization signal. As a result, all the terminals may have a common reference time. The synchronization signal used in this case may include a pulse signal, a Zadoff-Chu (ZC) sequence, an m-sequence, and a chirp signal. In such a method, it is assumed that terminals receive synchronization signals while sending a synchronization signal. In such a method, however, if terminals are crowded, synchronization signals transmitted by the terminals may be received overlapped. Accordingly, an error may occur in estimating the reception time of a synchronization signal, or the estimation of a reception time may be impossible. Furthermore, in an unlicensed frequency band, if the same radio channel is used by networks conforming to a different protocol in the same area, there is a problem in that a synchronization signal is not transmitted or a transmitted synchronization signal may not be received due to interference.